diff --git a/packages/vite/src/node/plugins/css.ts b/packages/vite/src/node/plugins/css.ts
index 4fd0bdf7f1f044..47276079f9e224 100644
--- a/packages/vite/src/node/plugins/css.ts
+++ b/packages/vite/src/node/plugins/css.ts
@@ -884,9 +884,13 @@ export function cssPostPlugin(config: ResolvedConfig): Plugin {
             .map((chunk) => [chunk.preliminaryFileName, chunk.fileName]),
         )
 
-        const pureCssChunkNames = [...pureCssChunks].map(
-          (pureCssChunk) => prelimaryNameToChunkMap[pureCssChunk.fileName],
-        )
+        // When running in watch mode the generateBundle is called once per output format
+        // in this case the `bundle` is not populated with the other output files
+        // but they are still in `pureCssChunks`.
+        // So we need to filter the names and only use those who are defined
+        const pureCssChunkNames = [...pureCssChunks]
+          .map((pureCssChunk) => prelimaryNameToChunkMap[pureCssChunk.fileName])
+          .filter(Boolean)
 
         const replaceEmptyChunk = getEmptyChunkReplacer(
           pureCssChunkNames,